In a typical glass melting furnace, glass batch ingredients are fed into a melting tank. The melting tank is provided with heating ports which are supplied with a combustible air-fuel mixture to issue combustion flames over the layer of batch ingredients fed onto the surface of previously melted glass in the tank. A temperature gradient exists between the surface of the glass, which is directly exposed to the applied flame heat, and the glass layer along the bottom of the tank. In order to obtain optimum melting conditions, it is desirable to maintain the temperature between the surface of the glass and the bottom layers of the glass as nearly uniform as possible, such as by setting up and controlling convection currents in the tank to continually mix the surface and bottom layers of glass. Bubbler systems have been successfully utilized in this regard to thermally and chemically homogenize the glass. Some representative bubbler systems are taught in U.S. Pat. Nos. 3,294,509 issued to Soubier; 3,853,524 issued to Schwenninger; 3,305,340 issued to Atkeson; 3,219,427 issued to Hymowitz; and 3,397,973 issued to Rough. Briefly, these bubbler systems employ a series of bubbler tubes arranged in various patterns usually along the refractory bottom or floor of the melting furnace. The bubbler tubes emit a gas into the molten glass and glass forming batch materials. The emitted gas enters the molten glass as a series of small bubbles at the bottom of the furnace. The bubbles expand under the influence of the high furnace and glass temperatures and rise toward the surface of molten glass where they burst and ordinarily are expelled from the furnace together with the gaseous products of combustion. Incident to the rising movements of these expanding gas bubbles, there is produced an agitation and stirring of the molten glass and unmelted batch materials. This agitation raises the relatively colder glass from the bottom of the furnace to the surface of the molten glass for exposure to heat. The displacement of the colder portions of the glass body causes displacement of upper hotter portions of the glass body into those normally colder lower portions of the glass body to thereby establish strong convection currents in the glass body. The convection currents act to continually mix the surface and bottom layers to minimize the temperature gradient through the molten glass body. Further, the bubbler action (i.e. agitation and stirring of the molten glass) promotes the expulsion of small gas bubbles or "seeds" which are entrapped within the molten glass during the melting process, thereby improving the quality of the glass. Moreover, this bubbler action achieves greater chemical as well as thermal homogeneity of the glass, a more economical utilization of the heat employed for the melting and refining operations and increased furnace melting capacity.
Although the hereinabove discussed bubbler systems have gained commercial acceptance, premature failure of the bubbler tubes due to oxidation and alkali-sulfate corrosion within the refractory mounting hole has inhibited the use of bubbler systems. This is so because often times when a bubbler tube is rendered inoperative, a new hole must be drilled through the refractory bottom to receive or accept a new bubbler tube. This is a time-consuming, costly, labor-intensive process. Further, each tube replacement procedure may disrupt the operational continuity of the melting furnace bubbler system which may result in the production of defective or inferior quality glass, because the loss of operational bubblers before replacement constitutes a threat to uniform heat distribution and glass homogeneity.
The prior art teaches the use of composite bubblers constructed of a molybdenum tube advantageously joined to a stainless steel tube as by brazing with nickel palladium. An inert gas such as nitrogen is moved through the bubblers to stir, agitate, and mix the molten glass. It has been determined that the portion of the molybdenum tube disposed within the hole in the refractory bottom of the melter tank is very susceptible to oxidative deterioration and alkali-sulfate corrosion. The molybdenum tube portion which extends into the molten glass is protected from oxidation and most alkali-sulfate corrosion because it is not exposed to oxygen as is the molybdenum tube portion which is disposed in the refractory bottom hole. High-grade, corrosion-resistant refractory metals such as inconel or other nickel alloy steel materials have also been unsuccessfully employed, due to relatively rapid deterioration in the hostile atmosphere of the refractory mounting hole. The temperature in the refractory mounting hole upper portion often exceeds 2,000.degree. F. (1,110.degree. C.). This high temperature condition, in combination with the highly oxidizing, alkali-sulfate atmosphere of the refractory bottom hole through which the composite bubbler is inserted, rapidly corrodes and deteriorates known materials. For example, stainless steel oxidizes at temperatures greater than about 1,700.degree. F. (950.degree. C.) and is also vulnerable to high temperature alkali-sulfate corrosion. U.S. Pat. No. 3,853,524 issued to Schwenninger teaches a monolithic molybdenum bubbler tube having a protective disilicide outer surface coating to protect the molybdenum tube from oxidizing in air. However, bull's-eye defects, stress cracks, handling damage, coating thickness variations, and bending stresses from improper installation have caused premature failure of the tubes within the refractory bottom, due to breakdown of the oxidation resistance of the disilicide coating.
Therefore, there presently exists a need for a bubbler capable of withstanding the hostile refractory mounting hole environment and which is more durable and longer lasting than presently available bubbler tubes.